Transformer Protection Relays (Buchholz Principle) - EMB Gmbh

Elektromotoren und 

Gerätebau Barleben GmbH 

Transformer Protection Relays 

(Buchholz Principle)

Elektromotoren und Gerätebau Barleben GmbH 

Table of contents 

2 

Page 

Company history 3 

1. Preface 4 

2. Design features 5 

3. Function 7 

3.1. Gas accumulation 7 

3.2. Insulating liquid loss 8 

3.3. Insulating liquid flow 8 

4. Tests 9 

5. Type list of single-float Buchholz relays 10 

5.1. Single-float Buchholz relays with threaded connection 10 

5.2. Single-float Buchholz relays with flanged connection 10 

6. Switching system design options for single-float Buchholz relays 11 

7. Type list of double-float Buchholz relays 12 

7.1. Double-float Buchholz relays with threaded connection 12 

7.2. Double-float Buchholz relays with flanged connection (round) 13 

7.3. Double-float Buchholz relays with flanged connection (square) 15 

7.4. Double-float Buchholz relays with geometrical flange dimensions 

according to Chinese norm 15 


according to former French norm 16 


according to former British standard 17 

8. Switching system design options for double-float Buchholz relays 18 

9. Technical data 22 

10. Special designs 23 

10.1. Explanations to codes 23 or 24 24 

10.2. Explanations to code 32 25 

11. Explanations to code 60 - Buchholz relay NM series 26 

11.1. Design features of Buchholz relay NM series 26 

11.2. Extra function of Buchholz relay NM series 26 

11.3. Analogue measuring unit - Analogue measurement of gas volume 27 

12. Ordering data/Type code 28 

12.1. Single-float Buchholz relays 28 

12.2. Double-float Buchholz relays 28 

13. Additional devices for Buchholz relays 30 

13.1. Gas sampling device ZG 1.2. 30 

13.2. Other additional devices for Buchholz relays 33 

14. Other protection devices 35

Company history 

Since its foundation the company has passed through an eventful history with regard to ownership, 

affiliation and change of name associated with such development. 

1921 Development of Buchholz relay by Max Buchholz 

1943 Branch of SIEMENS Magdeburg 

1948 VEB Elektromotorenwerk Barleben; VEM 

(state-owned firm) 

1951 VEB Starkstromanlagenbau Magdeburg 


1951 Start of manufacture of Buchholz relays in Barleben 

1965 Start of manufacture of monitoring relays for tap changers in Barleben 

1970 VEB Elektrotechnik und Gerätebau Magdeburg; EGEM 


1980 VEB Kombinat Elektromaschinenbau Dresden 

VEB Elektromotorenwerk Barleben; VEM; ELMO 


1990 VEM Antriebstechnik AG Dresden 

Elektromotorenwerk Barleben GmbH; VEM; ELMO 

(public limited company) 

1993 Elektromotoren und Gerätebau Barleben GmbH; EMB 

(privately owned company) 

2005 Start of manufacture of Buchholz relays NM series 

2009 New premises in Barleben 

Figure 1 - EMB company building 

3


1. Preface 

The Buchholz relay was developed in 1921 by Max Buchholz, Oberrat (senior councillor) at Preußische 

Elektrizitäts-AG (Prussian electricity supply company) in Kassel. Since that time it has been an 

important protection and monitoring device for insulating liquid-filled transformers with conservators 

and choke coils. It also allows separate monitoring of oil-filled bushings or cable terminal boxes. It 

is mounted in the cooling cycle of the appliance to be protected and responds to faults such as gas 

generation, loss of insulating liquid and excessively high flow rates of the latter. 

For transformers with hermetical closure by means of a membrane (rubber sack) in the conservator 

Buchholz relays can be used also as indicating devices (“air cell failure relay”) of this membrane. 

The Buchholz relay is suitable for open-air as well as indoor installations. 

Buchholz relays are available in a number of types which comply with norms and standards and 

meet special customer requirements. The type of relay to be used depends on the nominal rating 

and construction features of the appliance to be protected. Our range of products permits optimum 

flexibility. 

Elektromotoren und Gerätebau GmbH (EMB GmbH) provides more than 60 years experience in 

producing Buchholz relays and other protection devices for liquid-cooled and liquid-insulated appliances. 

It ranks among the most distinguished manufacturers of this type of equipment. EMB Buchholz 

relays comply with DIN EN 50216-2 and are known for their easy operation, high reliability and 

extremely long life. 

Our staff of highly qualified engineers and 

experienced skilled workers do their best to 

guarantee top quality high-precision products. 

The casings are machined on modern CNCcontrolled 

machine tools. All products are 

subjected to final inspection when all functions 

are checked using special test equipment. 

Profound experience and expertise are a 

sound basis for high product quality. 

Extensive references from reputed transformer 

manufacturers as well as other users are proof 

of the high qualitative level of the products. 

The company is certified to 

DIN EN ISO 9001/2008. 

4 

Figure 2 - DIN EN ISO 9001/2008 certificate

2. Design features 

Casing 

The casing is made of weather-resistant cast 

aluminium alloy provided with a paint coat. 

It is supplied either with screwed or flanged 

connection (1). The different casing designs 

available are shown in section 5 for singlefloat 

Buchholz relays and section 7 for doublefloat 

Buchholz relays, others are available on 

request. 

To check the switching system for proper 

function, the casing is provided with sightglasses 

(2). The sightglasses provided with scales 

permit reading of collected gas volume. 

The relays can be provided with hinged lids (3) 

to protect the sightglasses. 

Cover 

The upper section of the cover accommodates 

the terminal box (1). The test valve (2) and 

the test key covered by a cap nut (3) as well 

as a plate with instructions for actuating the 

test key (4) are arranged in front of the cover. 

The terminal box (1) accommodates the earth 

terminal (5) and up to eight bushings (6) for 

the terminals provided in the base of the cover. 

The number of these bushings determines the 

design of the switching systems in terms of 

type and quantity of the magnet contact tubes. 

The terminal box (1) is sealed by an aluminium 

cap (7) so that it is safe to touch and protected 

against pollution. If the cap is opened the graphic 

symbol and the connection diagram (8) 

are shown. The cable can be inserted through 

the cable gland (9). 

1 

3 

2 

1 

7 

8 

9 

5 

6 

1 

2 

3 

4 

Figure 3 - Casing with flanged connection 

Figure 4 - Casing with threaded connection 

Figure 5 - Cover with cap removed 

5


Switchgear 

The switchgear has the following main components: 

- Switching system 

- Frame 

- Mechanical testing unit. 

Whereas the single-float Buchholz relay has only one switching system, the double-float Buchholz 

relay has an upper and a lower switching system. Permanent magnet and float are rigidly linked 

forming an integrated whole that is movably fitted to the frame together with the mechanical testing 

unit and the magnet contact tube(s). 

Switchgear of a single-float Buchholz relay: 

- Float 

- Permanent magnet(s) 

- Magnet contact tube(s) 

- Damper 

The damper is held in its normal position by a magnet. 

Switchgear of a double-float Buchholz relay: 

- Upper float 

- Lower float 

- Permanent magnet(s) for upper float 

- Permanent magnet(s) for lower float 

- Magnet contact tube(s) for upper switching system 

- Magnet contact tube(s) for lower switching system 

- Damper 

The damper is held in its normal position by a magnet 

and acts on the lower switching system. 

6 

Figure 6 - Single-float Buchholz relay 

Figure 7 - Double-float Buchholz relay

3. Function 

The Buchholz relay is installed in the pipe between the tank of the device to be protected (transformer, 

reactor) and the conservator. During normal operation it is filled completely with insulating 

liquid. 

Due to buoyancy the float of the single-float relay and both floats of the double-float relay are at 

their top position. 

The upper and lower switching systems form a functional unit in the single-float Buchholz relay so 

that in the event of a fault the transformer is immediately disconnected from the power system. 

In the following the function of a Buchholz relay is explained using the example of a double-float 

Buchholz relay. If a fault occurs inside the transformer, the Buchholz relay responds as follows: 

3.1. Gas accumulation 

Fault: Free gas is available in the insulating liquid. 

Response: The gas in the liquid moves upwards, accumulates in the Buchholz relay and displaces 

the insulating liquid level. 

The moving float actuates a switch contact (magnet contact tube). An alarm signal is tripped. 

The lower float is not affected as from a certain gas volume the gas flows through a piping to the 

conservator. 

Oil Öl 

Gas 

Figure 8 - Gas accumulation 

7


3.2. Insulating liquid loss 

Fault: Insulating liquid loss due to leakage. 

Response: As the liquid level falls the top float moves downwards. An alarm is tripped. If the liquid 

loss continues, conservator and piping as well as the Buchholz relay will be emptied. 

As the liquid level falls, the lower float moves downwards. The moving float actuates a switch contact 

so that the transformer is disconnected. 

3.3. Insulating liquid flow 

Fault: A spontaneous incident generates a pressure wave moving in the direction of the conservator. 

Response: The liquid flow reaches the damper arranged in the liquid flow. If the flow rate exceeds 

the operating value of the damper, the latter moves in flow direction. 

Due to this movement a switch contact is actuated so that the transformer is disconnected. 

After release of the pressure wave the lower switching system returns to its starting position. 

Buchholz relays manufactured by EMB are equipped with a damper held by a permanent magnet. 

8 

Luft Air 

Oil Öl 

Oil Öl 

Figure 9 - Insulating liquid loss 

Figure 10 - Insulating liquid flow

4. Tests 

Each Buchholz relay is provided with a works-number that is specified on the test certificate and 

the name plate. The tests carried out on the Buchholz relay are recorded in the test certificate. 

- Dielectric strength test 

- Leakage test 

- Functional test 

- Flow test. 

Buchholz relays are delivered in cardboard boxes. 

For each relay delivered the following documents in the language agreed are proviced: 

- Operating instructions 

- Test certificate. 

The name plate covers the following information: 

Type 

works-no. 

6 digits 

switching IP code 

element 

S = normally-open 

Ö = normally-closed 

W = changed-over 

Figure 11 - Functional and leakage test 

date of manufacture 

(week/year) 

type code 

Figure 12 - Flow test 

9


5. Type list of single-float Buchholz relays 

5.1. Single-float Buchholz relays with threaded connection 

h1 

10 

Type 

Internal description 

DIN description 

01 

(AG 25) 

(CG 25) 

Type of connection 

Threaded 

connection 

G 1½ “ 

Figure 13 - Dimensional drawing, type 01 

Type 



02 

(AF 25/6) 

(-) 


Pipe 

diameter 

DN (mm) 

Flange dimensions 

(mm) 

Device dimensions 

(mm) 

d1 d2 d3 d4 d5 f l h1 h2 

5.2. Single-float Buchholz relays with flanged connection 

h1 

80 

h2 

80 

h2 

250 

300 

350 

03 

(AF 25/10) 

(-) 

25 

(AF 25) 

(-) 

250 

300 

350 

cm 3 

l 

cm 3 

l 

Weight 

(kg) 

Suited for 

transformer 

ratings of 

25 - - - - 16 185 170 62 3,1 ≤1600 KVA 

Pipe 

diameter 

DN (mm) 

Figure 14 - Dimensional drawing, type 02, 03, 25 

f 

f 


(mm) 


(mm) Weight 

(kg) 

d1 d2 d3 d4 d5 f l h1 h2 

Suited for 

transformer 

ratings of 

Flange 

4-hole 25 100 75 60 12 12 185 195 62 3,6 ≤1600 KVA 

Flange 

4-hole 25 115 85 68 14 16 200 205 62 4,0 ≤1600 KVA 

Flange 

4-hole 25 100 75 - 12 10 160 195 62 3,3 ≤1600 KVA 

d5 

170 

d1 

G11/2" 

170 

d1 

d4 

d3 

d2 

R80 

R80

6. Switching system design options for single-float Buchholz relays 

Magnet contact tubes are used as switching elements. These are normally-open (NO), 

normally-closed (NC) and change-over (CO) contacts. The magnet contact tube design can be 

derived from the last digit of the type code. For coding, see Ordering data/Type code under 

Section 12.1. on page 28. 

...1 ...2 ...3 ...4 ...5 ...6 

1 NO 1 NC 2 NO 2 NC 

13 

14 

21 

22 24 12 

11 

12 

13 

14 24 

23 11 

12 22 

1 NO and 

1 NC 

21 13 

11 

14 12 

...7 ...8 ...9 

2 CO 1 NO and 1 CO 1 NC and 1 CO 

11 

Explanation of symbols: 

2 

1 

4 

1 CO 

14 24 12 14 

23 

11 

Example: coding „ ... 6 “ 

Magnet contact tube(s) design 

Graphic symbol with terminal marking 

Connection diagram in terminal box 

The inner side of the cap accommodates a plate with the graphic symbol and the connection 

diagram. The schemes show the switching systems in their neutral position. The neutral 

position is the operating condition when the Buchholz relay is filled with insulating liquid up 

the required level and the device to be protected operates without any fault. 

21 

22 

12 

11 

2 

14 

1 CO 

1 

4 

11


7. Type list of double-float Buchholz relays 

7.1. Double-float Buchholz relays with threaded connection 

h1 

12 

Type 



04 

(BG 25) 

(DG 25) 

21 

(BG 25 S) 

(-) 


Threaded 

connection 

G 1½ “ 

Threaded 

connection 

G 1½ “ 


h1 

80 

h2 

80 

h2 

250 

300 

350 

cm 3 

l 

250 

300 

350 

cm 3 


l 

f 

f 

Pipe 

diameter 

DN (mm) 


(mm) 


(mm) Weight 

(kg) 

d1 d2 d3 d4 d5 f l h1 h2 h 3 

Suited for 

transformer 

ratings of 

25 - - - - 16 185 235 90 - 4,2 ≤5000 KVA 

25 - - - - 16 185 235 90 - 3,6 ≤5000 KVA 

170 

d1 

G11/2" 

150 

d1 

G11/2" 

R143

7.2. Double-float Buchholz relays with flanged connection (round) 

Type 



05 

(BF 25/6) 

(-) 

06 

(BF 25/10) 

(DR 25) 

23 

(BF 25/10 S) 

(-) 

07 

(BF 50/6) 

(-) 

08 

(BF 50/10) 

(DR 50) 

09 

(BF 80/10) 

(-) 

09-26. 

(BF80/10/8) 

(DR 80) 

24 

(BF 80/6) 

(-) 


Pipe 

diameter 

DN (mm) 


(mm) 


(mm) Weight 

(kg) 

d1 d2 d3 d4 d5 f l h1 h2 

Suited for 

transformer 

ratings of 

Flange 

4-hole 25 100 75 60 12 12 185 235 90 4,4 ≤5000 KVA 

Flange 

4-hole 25 115 85 68 14 18 200 235 90 4,8 ≤5000 KVA 

Flange 

4-hole 25 115 85 68 14 18 200 235 90 4,4 ≤5000 KVA 

Flange 

4-hole 50 140 110 90 14 12 185 235 80 4,6 

Flange 

4-hole 50 165 125 102 18 16 195 250 80 5,9 

≥5000 KVA 

≤10000 KVA 

≥5000 KVA 

≤10000 KVA 

Flange 

4-hole 80 200 160 138 18 15 195 265 80 6,2 ≥10000 KVA 

Flange 

8-hole 80 200 160 138 

18 

M16 

15 195 265 80 6,2 ≥10000 KVA 

Flange 

4-hole 80 190 150 130 18 15 195 260 80 6,0 ≥10000 KVA 

13


h1 

Figure 17 - Dimensional drawing, type 05, 06, 07, 08, 09, 24 

h1 


h1 

80 

h2 

80 

h2 

80 

h2 

Figure 19 - Dimensional drawing, type 09-26. 

14 

250 

300 

350 

cm 3 

l 

250 

300 

350 

cm 3 

l 

250 

300 

350 

cm 3 

l 

f 

f 

f 

d5 

150 

d1 

d4 

d3 

d2 

170 

d1 

d4 

d3 

d2 

170 

M16 

d1 

d4 

d3 

d2 

d5 

d5 

143 

R143

h1 

7.3. Double-float Buchholz relays with flanged connection (square) 

Type 



10 

(BF 80/Q) 

(DQ 80) 


Flange 

square 

4-hole 


Type 




Pipe 

diameter 

DN (mm) 


(mm) 


(mm) 

d1 b d3 d4 d5 f l h1 h2 

Weight 

(kg) 

Suited for 

transformer 

ratings of 

80 125 132 - 18 20 200 235 80 5,0 ≥10000 KVA 


according to Chinese norm 

63 

(BC 80) 

(QJ 80) 

Flange 

square 

4-hole 


Pipe 

diameter 

DN (mm) 


(mm) 


(mm) 

d1 b d3 d4 d5 f l h1 h2 

Weight 

(kg) 

Suited for 

transformer 

ratings of 

80 160 160 - 18 15 185 245 80 5,0 ≥10000 KVA 

Suitable for connection to Chinese butterfly valves (square flange). Other types on request. 

h1 

80 

h2 

80 

h2 

250 

300 

350 

cm 3 

250 

300 

350 

cm 3 

l 

l 

f 

f 

b 

b 

d3 

d3 

170 

d1 

b 

170 

d1 

b 

d5 

d5 

R143 

R143 

15



according to former French norm 

h1 

80 

h2 

16 

Type 



41 

(NF 25) 

(-) 

42 

(NF 50) 

(-) 

43 

(NF 80) 

(-) 

250 

300 

350 

cm 3 

l 


Pipe 

diameter 

DN (mm) 

Figure 22 - Dimensional drawing, type 41, 42, 43 


(mm) 


(mm) 

d1 d2 d3 d4 d5 f l h1 h2 

Weight 

(kg) 

Suited for 

transformer 

ratings of 

Flange 

4-hole 25 115 85 - 14 8 240 235 90 4,2 ≤5000 KVA 

Flange 

4-hole 50 165 125 - 18 11 240 250 80 5,1 

≥5000 KVA 

≤10000 KVA 

Flange 

4-hole 80 200 160 - 18 11 240 265 80 5,5 ≥10000 KVA 

b 

150 

d1 

d3 

b 

90° 

d5


according to former British standard 

h1 

Type 



51 

(BS 25) 

(-) 

52 

(BS 50) 

(-) 

53 

(BS 80) 

(-) 


Flange 

square 

4-hole 

Flange 

6-hole 

Flange 

6-hole 


h1 

80 

h2 

80 

h2 

250 

300 

350 

cm 3 

l 

250 

300 

350 

cm 3 

l 

Figure 24 - Dimensional drawing, type 52, 53 

f 

b 

Pipe 

diameter 

DN (mm) 


(mm) 


(mm) 

d1 d2 d3 d4 d5 f l h1 h2 

25 

50 

80 

- 

- 

140 

5,51 

160 

6,30 

150 

d1 

d3 

b 

170 

d1 

d3 

d2 

72 

2,83 

110 

4,33 

130 

5,12 

90° 

- 

- 

- 

- 

- 

- 

d5 

M10 

M10 

12 

0,47 

12 

0,47 

d5 

- 

- 

12 

0,47 

13 

0,51 

R143 

127 

5 

185 

7,28 

185 

7,28 

235 

9,25 

235 

9,25 

240 

9,45 

90 

3,54 

80 

3,15 

80 

3,15 

Weight 

(kg) 

Suited for 

transformer 

ratings of 

3,7 ≤ 5000 KVA 

4,8 

≥5000 KVA 

≤10000 KVA 

5,0 ≥10000 KVA 

17


8. Switching system design options for double-float Buchholz relays 

Magnet contact tubes are used as switching elements. These are normally-open (NO), 

normally-closed (NC) and change-over (CO) contacts. The magnet contact tube design can be 

derived from the last two digits of the type code. For coding, see Ordering data/Type code under 

Section 12.2. on page 2. 

18 

...11 BS 25...11 ...12 ...13 ...14 ...15 

Alarm Alarm Alarm Alarm Alarm Alarm 

1 NO 1 NO 1 NO 1 NO 1 NO 1 NO 

13 

14 

13 

14 

13 

14 

Disconnection Disconnection Disconnection Disconnection Disconnection Disconnection 

1 NO 1 NO 1 NC 2 NO 2 NC 

1 NO and 

1 NC 

23 

24 

23 

24 

11 

12 

23 

13 

14 

24 34 

33 11 

13 

14 

12 22 

21 23 

...16 ...17 ...19 ...21 

Alarm Alarm Alarm Alarm 

1 NO 1 NO 1 NO 1 NC 

23 

24 

13 

14 

Disconnection Disconnection Disconnection Disconnection 

12 

1 CO 2 CO 3 NO 1 NO 

11 

14 

21 

22 24 32 

31 

34 

23 

13 

14 

33 

24 34 

43 

44 

11 

12 

13 

14 

13 

14 

11 

24 12

...22 ...23 ...24 ...25 ...26 ...27 


1 NC 1 NC 1 NC 1 NC 1 NC 1 NC 

11 

12 

11 

12 

11 

12 


1 NC 2 NO 2 NC 

21 

22 

13 

14 24 

23 21 

22 32 

31 13 

11 

12 

1 NO and 

1 NC 

21 

14 22 

12 

21 

22 

1 CO 2 CO 

11 

14 

21 

22 24 32 

...31 ...32 ...33 ...34 ...35 ...36 


1 CO 1 CO 1 CO 1 CO 1 CO 1 CO 

21 

21 

21 

22 24 22 24 22 24 22 24 22 24 22 24 



13 

14 

11 

12 

33 

34 44 

43 31 

21 

32 42 

21 

1 NO and 

1 NC 

41 13 

11 

14 12 

12 

11 

12 

21 

1 CO 

11 

14 

31 

34 

19


20 

...41 ...42 ...43 ...44 ...45 ...46 


2 NO 2 NO 2 NO 2 NO 2 NO 2 NO 

13 

14 24 

23 13 

14 24 

23 13 

14 24 

23 13 

14 24 

23 13 

14 24 

23 23 

33 

24 34 



33 

34 

14 13 34 33 

11 

12 

33 

34 44 

43 11 

12 22 

1 NO and 

1 NC 

21 33 

11 

34 12 

1 CO 

...51 ...52 ...53 ...54 ...55 ...56 


1 NC and 

1 NO 

11 

12 14 

13 11 

1 NC and 

1 NO 

12 14 

13 11 

1 NC and 

1 NO 

12 14 

13 11 

1 NC and 

1 NO 

12 14 

13 11 

1 NC and 

1 NO 

12 14 

12 

13 21 

11 

14 

1 NC and 

1 NO 

23 

22 24 



23 

24 

21 

22 

23 

24 34 

33 21 

22 32 

31 21 

1 NC and 

1 NO 

23 

22 24 

12 

1 CO 

11 

14

Explanation of symbols: Example: coding „ ...1 2 “ 

Magnet contact tube(s) design 

Upper switching system - Alarm 

Lower switching system - Disconnection 

13 

14 

11 

12 

Alarm 

1 NO 

Disconnection 

1 NC 

Graphic symbol with terminal marking 

Connection diagram in terminal box 

The inner side of the cap accommodates a plate with the graphic symbol and the connection 

diagram. The schemes show the switching systems in their neutral position. The neutral 

position is the operating condition when the Buchholz relay is filled with insulating liquid up 

the required level and the device to be protected operates without any fault. 

21


9. Technical data 

The technical data listed in Table 1 are applicable to all Buchholz relays manufactured by EMB. 

EMB Buchholz relays are in compliance with DIN EN 50216-2. Options available are specified in 

tables of Section 10 on page 23. These special designs are coded using the respective code when 

ordering Buchholz relays. 

Parameter Data Notes 

Voltage AC 12 V - max. 250 V 

DC 12 V - max. 250 V 

Current AC 0.01 A - max. 2 A 

DC 0.01 A - max. 2 A 

Switching capacity AC max. 400 VA 

DC max. 250 W 

Dielectric strength AC 2500 V 

AC 2000 V (normally-open contact) 

AC 1000 V (change-over contact) 

Temperature range: 

- Ambient temperature 

- Operating range 

* Temperature of the insulating liquid 

22 

* Viscosity of the insulating liquid 

- 40 °C to + 55 °C 

- 40 °F to + 131 °F 

- 40 °C to + 115 °C 

- 40 °F to + 239 °F 

1 mm 2 /s to 1100 mm 2 /s 

cos φ > 0,5 

L/R < 40 ms 

between electric circuit and earth 

between open contacts 

Climatic testing acc. to 

DIN EN 60068-2-78: 2002-09 

Others on request: 

extreme frigid open-air conditions 

below - 40 °C or offshore 

Resistance against vibration Class 4M6 Classification acc. to 

DIN EN 60721-3-4 

Resistance to pressure 0.25 MPa 

Resistance to vacuum < 2.5 kPa 

Insensitivity to magnetic fields 25 mT Static magnetic field of any 

direction and polarity 

Switching system: 

- Number of switching contacts 

- Switching element 

- Damper 

Response time of damper 

Response of switching system in 

case of: 

- Gas accumulation 

- Insulating liquid flow 

Pipe diameter DN of 

25 mm, 50 mm or 80 mm 

1 

Magnet contact tube 

Held by magnets 

< 0.1 s 

200 cm 3 to 300 cm 3 

min. 0.65 to max. 3.00 m/s 

+/- 15% 

More on request 

For possible data see Ordering 

data/Type code under Section 12 

on pages 28 and 29. 

Cable gland M20x1.5; M25x1.5 Others on request 

Nominal installation position 0° to 5° Ascending towards conservator 

IP code IP 54; IP 56; IP 66 Others on request 

Casing colour Two-component texture paint On polyurethane basis

10. Special designs 

Buchholz relay NM series 

Explanation Code 

NM series - Buchholz relay with analogue measurement of the gas volume 

(only double-float Buchholz relays) 60 

Casing colour 

Casing colour RAL 7001 (silver-grey) 41 

Casing colour RAL 7012 (basalt-grey) 42 

Casing colour RAL 7022 (umber-grey) 43 

Casing colour RAL 7033 (cement-grey) 44 

Casing colour RAL 7038 (agate-grey) 45 

Casing colour RAL 7035 (light-grey) 46 

Casing colour RAL 7016 (anthracite-grey) 47 

Casing colour RAL 9002 (grey-white) 48 

Casing colour RAL 7032 (siliceous-grey) 49 

Climate-proof version 

Climate-proof version (extreme frigid open-air conditions below - 40 °C) 34 

Climate-proof version (offshore) 36 

IP code 

IP code IP 56 38 

IP code IP 66 39 

Insulating liquid 

Insulating liquid silicone oil 20 

Insulating liquid based on esther 21 

Casing 

With oil drain plug (only double-float Buchholz relays) 28 

With premounted Harting-Connector 

(The option is indicated by a letter after the code.) 59 

Switching system (for design options see page 11 or pages 18-20) 

Upper switching system equipped with two magnet contact tubes 35 

Lower switching system equipped with two magnet contact tubes 25 

Upper and lower switching system each equipped with two magnet contact tubes 33 

Lower switching system equipped with three magnet contact tubes 

Testing of the switching systems by means of compressed-air and test key 

99 

(only double-float Buchholz relays) 32 

Damper held in response position (only double-float Buchholz relays) 23 

Special design approved by RWE, Germany (only double-float Buchholz relays) 24 

Special request 

Special request (on special agreement with customer) 29 

23


For engineering reasons the following special designs cannot be combined in the same device: 

10.1. Explanations to codes 23 or 24 

Buchholz relays with the feature „damper held in response position“ are designed such that the 

damper after it was operated due to an unacceptable high flow rate of the insulating liquid is 

locked in its position and, hence is kept in this position even after the flow rate has been reduced. 

This means that the signal generated is maintained. 

The damper has to be unlocked manually by turning the test key anticlockwise. When unlocking 

the damper, also check the insulating liquid level in the Buchholz relay. Bleed the Buchholz relay, 

if required. 

24 

Code combination Code combination Code combination 

60 - 32 32 - 23, 24 33 - 23, 24 

60 - 33 32 - 25 33 - 99 

60 - 34 32 - 33 

60 - 35 32 - 35 35 - 23, 24 

60 - 36 

60 - 59 

32 - 99 35 - 99 

60 - 99 99 - 23, 24

10.2. Explanations to code 32 

For Buchholz relays provided additionally with an air nipple (code 32), the mechanical function of 

the two switching systems can be tested by means of test key (1), and the upper switching system 

(alarm) can be tested by pumping in air via the test valve (2) using a suitable test pump. Additionally, 

the switching systems can be tested pneumatically. To this end, air is supplied via an air 

supply nipple (3) provided with a check valve. Perform the test while the Buchholz relay is filled with 

insulating liquid up to the required level. 

Pneumatic test of the upper switching system (alarm) using compressed air: 

Air is introduced slowly into the Buchholz relay through the air supply nipple and the pipe air until 

the alarm contact is made when the upper float is lowered. 

Pneumatic test of the lower switching system (disconnection) using compressed air: 

Through the air supply nipple and the pipe air is applied suddenly to the damper. When the damper 

responds the disconnection contact is made. 

After any test using air, bleed the Buchholz relay through the test valve. 

The functional test using compressed air results from British Standard B.E.B.S. T2 of 1966. 

Figure 25 - Cover with additional air supply nipple 

2 3 1 

25


11. Explanations to code 60 - Buchholz relay NM series 

11.1. Design features of Buchholz relay NM series 

The principal construction of the Buchholz relay with floats and damper as well as their electromechanical 

functions have been maintained. 

The Buchholz relay of series NM is equipped additionally with a capacitive sensor. The sensor is 

installed in the cover of the Buchholz relay. The cap of the terminal box accommodates the electronic 

amplifier of the measuring unit. Sensor and amplifier are connected by a shielded cable 

in three-wire technology of 15 m length with a cable connector. This cable is used for the supply 

voltage and the output signal. 

Figure 26 shows the arrangement of the measuring unit using the example of a Buchholz relay BF 

80/10/8. Apart from the dust hood and the cap of the terminal box which are higher by approx. 30 

mm, the installation dimensions of the relay have not been changed. Hence, relays with analogue 

measuring units can be installed in existing systems. 

291 

Figure 26 - Dimensional drawing Buchholz relay NM series type 09-26. (BF 80/10/8) 

11.2. Extra function of Buchholz relay NM series 

The standard Buchholz relay detects undissolved gases in the insulating liquid and indicates their 

presence when a specified threshold is exceeded, meaning that up to a certain gas volume, no 

signal is generated. Neither is it possible to gather information about the length of time it takes for 

gas to accumulate. 

The process of the generation of unsolved gases in the insulating liquid over time is a very important 

criterion for the evaluation of the fault as the quantity and composition of the fault gases 

depend on the kind and quantity of the energy of the underlying fault. Spontaneous and energyrich 

faults cause large gas quantities within a short period of time, whilst small volumes of gas are 

generated in the case of minor and insidious faults. 

The Buchholz relay of NM series allows continuous and analogue gas volume measurement so 

that gases which have accumulated in the relay are detected at an early stage, information about 

the generation of the gases obtained and the conditions provided for the analysis of the fault at an 

early stage. 

26 

109 

80 

195 

15 

138 

200 

M16 

170 

80 

160 

18 

R143

The extra function of the NM series relays is ensured by a capacitive sensor and the appropriate 

electronic components. Supply voltage of this assembly is DC 24 V. This voltage must be made 

available by the user. The output signal of the measuring unit is a standard current signal of 

DC 4 to 20 mA. It depends on the user how and in what form this signal is processed. 

DC 4 - 20 mA 

Monitoringsystem 

Extra function 

of the Buchholz 

relay NM series 

DC 24 V 

Conversion and 

processing 

Figure 27 - Extra function of Buchholz relay NM series 

11.3. Analogue measuring unit - Analogue measurement of gas volume 

The measured value is based on capacitance variation of the sensor caused by variation of the 

insulating liquid level in the Buchholz relay. 

Analogue measurement of the gas volume is possible between 50 cm 3 and 300 cm 3 . Lower gas 

volumes cannot be measured reliably because of high inaccuracies. Measurements beyond this 

range are not necessary as in this case the upper switching system will respond. Besides, the 

construction of the Buchholz relay (larger gas volumes escape towards the conservator) does not 

allow such measurements. The operating point of the upper switching system (upper float) is a gas 

volume of 200-300 cm 3 . 

Fault: Free gas is available in the insulating liquid. 

Standard function 

of the Buchholz relay 

DC 12 V - max. 250 V 

DC 0,01 A - max. 2 A 

Disconnection Alarm 

Response: The gas in the liquid moves upwards, accumulates in the Buchholz relay and displaces 

the insulating liquid. This decreases the level of insulating liquid. Liquid level changes lead to variation 

of the capacity of the sensor. This change is converted into an analogue current signal. 

It should be noted that for design reasons the current value of the sensor remains relatively constant 

up to a gas volume of approx. 50 cm 3 . The function equation will provide the actual volume 

only when the current signal becomes smaller and, hence the calculated volume noticeably larger. 

27


12. Ordering data/Type code 

For placing orders, please, use the following key: 

12.1. Single-float Buchholz relays 

0 = not occupied 

Type 

(see Section 5) 

Special design 


Damper setting (m/s) 

01 = 0,65 +/- 15% 

02 = 1,00 +/- 15% 

03 = 1,50 +/- 15% 

Contact setting of 

switching system 

1 = one NO contact 

2 = one NC contact 

3 = two NO contacts 

4 = two NC contacts 

5 = one NO and one NC contact 

6 = one CO contact 

7 = two CO contacts 

8 = one NO and one CO contact 

9 = one NC and one CO contact 

12.2. Double-float Buchholz relays 

Ordering example for double-float Buchholz relays: 

XX XX. XX. X 0 X 

For switching system design options for singlefloat 

Buchholz relays, see Section 6 on page 11. 

You need a Buchholz relay of type BF 80/Q. The damper should respond at a flow rate of 1.50 m/s. 

The upper switching system should be equipped with one switching element (magnet contact tube) 

and the lower with two switching elements (magnet contact tubes). The upper switching element 

should be designed as one normally open contact, and the lower as two normally open contacts. 

The device should be delivered in colour RAL 7033 and should have one oil drain plug. 

Based on the data on page 29 the relay ordered has the following 

Type code: 10-25.28.44.-0313 

Legend: 

NO= normally-open contact 

NC = normally-closed contact 

CO = change-over contact 

Explanation: 10 = Double-float Buchholz relay type 10 (BF 80/Q) 

25 = Lower switching system equipped with 2 magnet contact tubes 

28 = With oil drain plug 

44 = Casing colour RAL 7033 (cement-grey) 

03 = Damper setting: 1.50 m/s +/- 15 % 

1 = Contact setting of upper switching system: 1 NO 

3 = Contact setting of lower switching system: 2 NO 

28

Double-float Buchholz relays 

Type 


Special design 


Damper setting (m/s) 

01 = 0,65 +/- 15% 11 = 1,20 +/- 15% 

02 = 1,00 +/- 15% 12 = 1,25 +/- 15% 

03 = 1,50 +/- 15% 13 = 1,30 +/- 15% 

04 = 2,00 +/- 15% 14 = 1,40 +/- 15% 

05 = 2,50 +/- 15% 15 = 1,80 +/- 15% 

06 = 3,00 +/- 15% 


upper switching system 

(alarm) 





5 = one NC and one NO contact 


lower switching system 

(disconnection) 




4 = two NC contacts 

5 = one NO and one NC contact 


7 = two CO contacts 

9 = three NO contacts 

XX XX. XX. XX X X 

Legend: 

NO= normally-open contact 

NC = normally-closed contact 

CO = change-over contact 

For switching system design options for double-float 

Buchholz relays, see Section 8 on pages 18-20. 

29


13. Additional devices for Buchholz relays 

13.1. Gas sampling device ZG 1.2. 

The gas sampling device is mounted on the transformer and connected to the Buchholz relay 

by means of a pipe. It allows sampling of the relay gas at normal operating level. 

The length of the pipe can be selected by the customer (please, see page 31). 

The device can be delivered with a lockable box. 

Figure 28 - Gas sampling device ZG 1.2. Figure 29 - Gas sampling device ZG 1.2. 

in lockable box 

Figure 30 - Pipe of gas sampling device ZG 1.2. Figure 31 - Sightglass cover for 

gas sampling device ZG 1.2. 

30

Technical data 

Parameter Data Notes 

Gas outlet opening G 1/8“ Others on request 

Oil outlet opening G 1/8“ Others on request 

Temperature range: 

- Ambient temperature 

- Operating range 

* Temperature of the insulating liquid 

- 40 °C to + 55 °C 

- 40 °F to + 131 °F 

- 40 °C to + 115 °C 

- 40 °F to + 239 °F 

* Viscosity of the insulating liquid 1 mm2 /s to 1100 mm2 Others on request: 

extreme frigid open-air conditions 

below - 40 °C or offshore 

/s 

Weight without piping 2,2 kg 

Dimensions of pipe Ø 6x1 copper 

Length of pipe < 20 m As requested by customer 

Casing colour Two-component texture paint On polyurethane basis 

Special designs: 

Colour 

Explanation Code 

Casing colour RAL 7001 (silver-grey) 41 

Casing colour RAL 7012 (basalt-grey) 42 

Casing colour RAL 7022 (umber-grey) 43 

Casing colour RAL 7033 (cement-grey) 44 

Casing colour RAL 7038 (agate-grey) 45 

Casing colour RAL 7035 (light-grey) 46 

Casing colour RAL 7016 (anthracite-grey) 47 

Casing colour RAL 9002 (grey-white) 48 

Casing colour RAL 7032 (siliceous-grey) 49 

Climate-proof version 

Climate-proof version (extreme frigid open-air conditions below - 40 °C) 34 

Climate-proof version (offshore) 36 

Insulating liquid 

Insulating liquid silicone oil 20 

Insulating liquid based on esther 21 

Special request 

Special request (on special agreement with customer) 29 

31


210 

15 

Figure 32 - Dimensional drawing, type ZG 1.2. 

32 

90 XX. XX. X XX,XXm 

Type for 

ZG 1.2. 

125 

Ordering data/Type code of gas sampling device ZG 1.2. 

Special designs 

(see page 31) 

0 = without box 

1 = with box 

Ordering example of gas sampling device ZG 1.2.: 

9 

30 

93 

Length of pipe 

in meters 

Type code: 90-34.44.-0-10,50m 

Explanation: 90 = Gas sampling deviceZG 1.2. 

34 = Climate-proof version (extreme frigid open-air conditions below - 40 °C) 

44 = Casing colour RAL 7033 (cement-grey) 

0 = Without box 

10,50m = Length of pipe 10,50 m 

10

13.2. Other additional devices for Buchholz relays 

Gas testing device ZG 3.1. 

The gas testing device is used to test the gas accumulated in the Buchholz 

relay. It can be installed either directly on the test valve of the Buchholz relay 

or on the gas outlet tap of the gas sampling device. The Buchholz gas flows 

through two different chemical solutions and its colour reactions indicate the 

nature of the fault. 

Use of the gas testing device is no substitute for a gas chromatographic 

analysis. 

Reflux lock ZG 4.1. 

The device prevents insulating liquid from flowing into the gas testing device. 

The device is installed between the Buchholz relay or gas sampling device 

and the gas testing device. 

Test pump ZG 5.1. and ZG 5.2. 

The test pump checks the functioning of the upper switching system (alarm) 

of the Buchholz relay by pumping in air. The test can be performed directly 

on the Buchholz relay. For that purpose, the test pump is connected to 

the test valve of the Buchholz relay. When the test is performed via the gas 

sampling device, the test pump is connected to the gas outlet tap of the gas 

sampling device. 

- ZG 5.1. manually operated 

- ZG 5.2. pedal-operated 

Oil sampling device ZG 6.1. 

The oil sampling device is connected to the Buchholz relay via a pipe and 

is used to take oil samples from the Buchholz relay (suitable for use with 

Buchholz relays with an oil drain plug). The pipe is supplied to the customer’s 

specifications. 

33


34 

Buchholz gas sampler BGS 

The Buchholz gas sampler provides a safe method of taking and transporting 

gas samples from the Buchholz relay or the gas sampling device. Its capacity 

is 100 ml. 

Buchholz gas tester BGT 

The Buchholz gas tester is used to measure the hydrogen concentration of 

the Buchholz gases. The measurement can be performed directly where the 

sample is taken. 

For other gas testing equipment, please, contact us.

14. Other protection devices 

Monitoring relay for tap changers ÜRF 25, ÜRF 25/10, ÜRF 25/10-26 

The monitoring relay for tap changers, also known as the protection relay for 

tap changers or oil flow relay, is a monitoring device for insulating liquid-filled 

tap changers with conservator. It protects the tap changer and the transformer 

from damage. The monitoring relay responds to excessive oil flow in the 

direction of the conservator and generates a signal disconnecting the tap 

changer and the transformer immediately from voltage supply. 

Pipe diameter DN: 25 mm (1“) 

Type of connection: threaded or flanged 

Oil flow indicator SG 25, SF 25, SF 25/10 

The oil flow indicator is a protective relay monitoring the circulating oil lubrication 

or circulating oil cooling of machines and equipment. It indicates faults 

in circulating oil systems or shuts down the machine or equipment, thus preventing 

damage. The flow indicator operates at a very low service pressure, 

so it can be installed in an oil return pipe where the oil flow is ensured by the 

sloping pipe. 

Pipe diameter DN: 25 mm (1“) 

Type of connection: threaded or flanged 

For further information, please ask for special reference material. 

35

EMB GmbH 

Otto-von-Guericke-Allee 12 

D-39179 Barleben | Germany 

Phone: +49 39203 790 

Fax: +49 39203 5330 

Email: info@emb-online.de 

Website: www.emb-online.de 

www.buchholzrelay.com 

Due to technical improvement of our products, the information contained in this catalogue is subject to change without notice. 

We would like to apologize for any printing errors which have not been found despite of intensive proof-reading. 

Edition: Catalogue Buchholz relays KA 01/01/12/02 English 

Elektromotoren und 

Gerätebau Barleben GmbH 

Köln 

Frankfurt 

Hamburg 

Hannover 

Barleben 

Magdeburg 

München 

Leipzig 

Berlin

Transformer Protection Relays (Buchholz Principle) - EMB Gmbh

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